A novel and efficient method for synthetic carbohydrate conjugate vaccine preparation: synthesis of sialyl Tn-KLH conjugate using a 4-(4-N-maleimidomethyl) cyclohexane-1-carboxyl hydrazide (MMCCH) linker arm

Hib Vaccines: Past, Present, and Future Perspectives

Haemophilus influenzae type b (Hib) causes many severe diseases, including epiglottitis, pneumonia, sepsis, and meningitis. In developed countries, the annual incidence of meningitis caused by bacteria is approximately 5–10 cases per population of 100,000. The Hib conjugate vaccine is considered protective and safe. Adjuvants, molecules that can enhance and/or regulate the fundamental immunogenicity of an antigen, comprise a wide range of diverse compounds. While earlier developments of adjuvants created effective products, there is still a need to create new generations, rationally designed based on recent discoveries in immunology, mainly in innate immunity. Many factors may play a role in the immunogenicity of Hib conjugate vaccines, such as the polysaccharides and proteins carrier used in vaccine construction, as well as the method of conjugation. A Hib conjugate vaccine has been constructed via chemical synthesis of a Hib saccharide antigen. Two models of carbohydrate-protein conjugate have been established, the single ended model (terminal amination-single method) and cross-linked lattice matrix (dual amination method). Increased knowledge in the fields of immunology, molecular biology, glycobiology, glycoimmunology, and the biology of infectious microorganisms has led to a dramatic increase in vaccine efficacy.

Adjuvanticity of a recombinant calreticulin fragment in assisting anti-β-glucan IgG responses in T cell-deficient mice

Polysaccharide-encapsulated fungi are the chief source of diseases in immunocompromised hosts such as those infected with human immunodeficiency virus or neutropenia patients. Currently available polysaccharide-protein conjugate vaccines are mainly T cell dependent and are usually ineffective in weakened immune systems. In this study, laminarin, a well-characterized β-1,3-glucan, was conjugated with a prokaryotically expressed recombinant fragment (amino acids [aa] 39 to 272) of calreticulin (rCRT/39-272), which exhibits extraordinarily potent immunogenicity and adjuvanticity in experimental animals. The resultant conjugate reserves the immunostimulatory effect of rCRT/39-272 on naïve murine B cells and is capable of eliciting anti-β-glucan IgG (mostly IgG1) responses in not only BALB/c mice but also athymic nude mice. Laminarin-CRT-induced mouse antibodies (Abs) are able to bind with Candida albicans and inhibit its growth in vitro. In addition, vaccination with laminarin-CRT partially protects mice from lethal C. albicans challenge. These results imply that rCRT/39-272 could be used as an ideal carrier or adjuvant for carbohydrate vaccines aimed at inducing or boosting IgG responses to fungal infections in immunodeficient hosts.

Recent advances in developing synthetic carbohydrate-based vaccines for cancer immunotherapies

Cancer cells can often be distinguished from healthy cells by the expression of unique carbohydrate sequences decorating the cell surface as a result of aberrant glycosyltransferase activity occurring within the cell; these unusual carbohydrates can be used as valuable immunological targets in modern vaccine designs to raise carbohydrate-specific antibodies. Many tumor antigens (e.g., GM2, Ley, globo H, sialyl Tn and TF) have been identified to date in a variety of cancers. Unfortunately, carbohydrates alone evoke poor immunogenicity, owing to their lack of ability in inducing T-cell-dependent immune responses. In order to enhance their immunogenicity and promote long-lasting immune responses, carbohydrates are often chemically modified to link to an immunogenic protein or peptide fragment for eliciting T-cell-dependent responses. This review will present a summary of efforts and advancements made to date on creating carbohydrate-based anticancer vaccines, and will include novel approaches to overcoming the poor immunogenicity of carbohydrate-based vaccines.

Anti-idiotypic antibody mimicking a T-antigen-specific lectin inhibits human epithelial tumor cell proliferation

Cancer-associated mucins show frequent alterations of oligosaccharide chain profile. Terminal structures may be deleted, thereby exposing normally 'cryptic' structures such as Tn (GalNAcα-O-Ser/Thr) and T antigen (Galβ1-3GalNAcα-O-Ser/Thr). Overexpression of these commonly hidden glycoforms, and reduced level of naturally occurring anti-T or anti-Tn antibodies, is associated with epithelial tumor progression and aggressiveness. The lectin from the common edible mushroom Agaricus bisporus (ABL) shows high affinity binding to T antigen, and reversible noncytotoxic inhibitory effect on epithelial tumor cell proliferation. The aim of this study was to induce immune response with tumor-associated glycan specificity and biological activity similar to those of ABL. An anti-idiotypic (Id) antibody strategy was developed using ABL as first template. ABL was purified by affinity chromatography and assayed as immunogen in rabbit. Rabbit IgG was purified from anti-ABL serum using a protein G column, and specific anti-ABL IgG was obtained by affinity chromatography using immobilized ABL. Affinity-purified anti-ABL IgG contained an antibody fraction that recognizes the carbohydrate-binding site of ABL. This IgG was used as immunogen in mouse to yield anti-Id antibody recognizing tumor-associated glycans such as Tn and T antigen. Competitive assays showed that α-anomeric GalNAc is the main binding subsite of anti-Id antibody in glycan recognition. Anti-Id antibody bound human epithelial tumor cells, as shown by cell enzyme-linked immunosorbent assay and immunofluorescence. Anti-Id antibody raised by immunization with affinity-purified anti-ABL IgG had antiproliferative effect on human epithelial tumor cells through apoptosis induction similar to that of ABL. The anti-Id immune response developed here has potential application in cancer therapy.

Immunotherapy for cancer: synthetic carbohydrate-based vaccines

Aberrant glycosylation of glycoproteins and glycolipids of cancer cells, which correlates with poor survival rates, is being exploited for the development of immunotherapies for cancer. In particular, advances in the knowledge of cooperation between the innate and adaptive system combined with the implementation of efficient synthetic methods for assembly of oligosaccharides and glycopeptides is providing avenues for the rationale design of vaccine candidates. In this respect, fully synthetic vaccine candidates show great promise because they incorporate only those elements requires for relevant immune responses, and hence do not suffer from immune suppression observed with classical carbohydrate-protein conjugate vaccines. Such vaccines are chemically well-defined and it is to be expected that they can be produced in a reproducible fashion. In this feature article, recent advances in the development of fully synthetic sub-unit carbohydrate-based cancer vaccines will be discussed.

Synthetic and immunological studies of 5'-N-phenylacetyl sTn to develop carbohydrate-based cancer vaccines and to explore the impacts of linkage between carbohydrate antigens and carrier proteins

5'- N-Phenylacetyl sTn (sTnNPhAc), an unnatural derivative of sTn antigen expressed by many tumors, and its alpha-linked protein conjugates were prepared and investigated to explore glycoconjugate cancer vaccines. sTnNPhAcalpha-KLH elicited a robust T cell dependent immunity. The antiserum derived from sTnNPhAcalpha- or sTnNPhAcbeta-KLH-inoculated mice was similarly reactive to sTnNPhAcalpha and sTnNPhAcbeta but showed very little reactivity to sTn, NeuNPhAcalpha(2,3)GalNAc--a regioisomer of sTnNPhAc, isolated phenylacetyl group, and the linker employed to conjugate sTnNPhAc and carrier protein. It was concluded that the sTnNPhAc-elicited immunity was specific for the whole antigen rather than the phenylacetyl group or other partial structures of sTnNPhAc and that the reducing end configuration or linkage of sTnNPhAc did not affect its immunological identity. It was also concluded that a new linker designed to conjugate carbohydrates and proteins did not provoke any immune reaction and that the linker, as well as the associated new and convenient coupling strategy, can be safely used for the development of glycoconjugate vaccines.

A polyvalent vaccine for high-risk prostate patients: "are more antigens better?"

We have shown the immunogenicity and safety of synthetic carbohydrate vaccines when conjugated to the carrier keyhole limpet hemocyanin (KLH) and given with the adjuvant, QS-21, in patients with biochemically relapsed prostate cancer. To determine whether immune response could be further enhanced with stimulation by multiple antigens, a hexavalent vaccine was prepared using previously determined doses and administered in a Phase II setting to 30 high-risk patients. The hexavalent vaccine included GM2, Globo H, Lewis(y), glycosylated MUC-1-32mer and Tn and TF in a clustered formation, conjugated to KLH and mixed with QS-21. Eight vaccinations were administered over 13 months. All 30 patients had significant elevations in antibody titers to at least two of the six antigens; 22 patients had increased reactivity with FACS. These serologic responses were lower than that seen previously in patients treated with the respective monovalent vaccines. The reciprocal median combined IgM and IgG antibody titers with ELISA against MUC1, Tn, TF, globo H and GM2 for these 30 patients were 640, 80, 120, 40 and 0, compared to 1280, 640, 1280, 320 and 160 seen in patients receiving individual monovalent vaccines. This hexavalent vaccine of synthetic "self" antigens broke immunologic tolerance against two or more antigens in all 30 vaccinated patients, was safe, but antibody titers against several of the antigens were lower than those seen in individual monovalent trials. No impact on PSA slope was detected. We address the relevance of the multivalent approach for prostate cancer treatment.

Improving the antigenicity of sTn antigen by modification of its sialic acid residue for development of glycoconjugate cancer vaccines

Sialyl Tn (sTn) antigen is a sialylated disaccharide abundantly expressed by many tumors. To search for effective cancer immunotherapies based on sTn antigen, we designed and synthesized a series of unnatural N-acyl derivatives of sTn and studied their immunological properties. For this purpose, an efficient method was developed to synthesize the natural and unnatural forms of sTn antigen and their protein conjugates. The resultant glycoconjugates were used to immunize C57BL/6 mice, and the immune response was assessed by enzyme-linked immunosorbent assay (ELISA). Whereas the keyhole limpet hemocyanin (KLH) conjugate of sTn elicited low levels of IgM antibodies, the KLH conjugates of N-iso-butanoyl sTn and N-phenylacetyl sTn, especially the latter, induced high titers of antigen-specific IgG antibodies, showing a T-cell-dependent response that is critical for the antitumor activity. The results suggest that the modified forms of sTn, especially N-phenylacetyl sTn, have improved antigenicity and promising immunological properties for use as cancer vaccines.

Toward a Carbohydrate-Based HIV-1 Vaccine: Synthesis and Immunological Studies of Oligomannose-Containing Glycoconjugates

Human antibody 2G12 is a broadly neutralizing antibody that exerts its anti-HIV activity by targeting a novel oligomannose cluster on HIV-1 gp120. It was previously demonstrated that synthetic oligomannose clusters could mimic the carbohydrate epitope of 2G12 and showed enhanced antigenicity (Wang L. X. et al. (2004) Chem.Biol. 11, 127). This paper describes the synthesis of oligomannose-containing glycoconjugates that include either a carrier protein or a universal T-helper epitope peptide to provide an effective immunogen. It was shown that the synthetic neoglycoconjugates containing oligomannose clusters could be recognized by the human antibody 2G12. Rabbit immunization studies revealed that only a small fraction of antibodies raised by the glycoconjugates was directed to the carbohydrate antigens, with the majority of the IgG type antibodies being directed to the linkers in the conjugates. The anti-sera showed weak cross-reactivity to HIV-1 gp120.

Chemoselective Assembly and Immunological Evaluation of Multiepitopic Glycoconjugates Bearing Clustered Tn Antigen as Synthetic Anticancer Vaccines

In this paper we investigated the use of regioselectively addressable functionalized templates (RAFTs) as new scaffolds for the design of anticancer vaccine candidates. We report the synthesis of well-defined multiepitopic RAFT scaffolds and their immunological evaluation. These conjugates exhibit clustered Tn analogue as tumor-associated carbohydrate antigen (TACA, B-cell epitope) and the CD4+ helper T-cell peptide from the type 1 poliovirus. The saccharidic and peptidic epitopes were both synthesized separately and combined regioselectively to the RAFT core using a sequential oxime bond formation strategy. B- and T-antigenicity and immunogenicity of the vaccine candidates were investigated in vitro and in vivo. These studies clearly demonstrate that the saccharidic part of the conjugates is recognized by Tn-specific monoclonal antibodies. Moreover, the antibodies elicited by immunization of mice with our vaccine candidates recognize the native form of Tn epitope expressed on human tumor cells. Together with oxime ligation technique, these results suggest that the RAFT scaffold provides a promising and suitable tool for engineering potent synthetic anticancer vaccine.

Carbohydrate vaccines as immunotherapy for cancer

Carbohydrates have established themselves as the most clinically relevant antigens of those tested and subsequently developed for vaccines against infectious diseases. However, in cancer patients, many of the defined carbohydrate antigens are really altered 'self' antigens and for unclear reasons, the body does not react to them immunologically. Although these self antigens have been found to be potentially suitable targets for immune recognition and killing, the development of vaccines for cancer treatment is actually more challenging compared with those for infectious diseases mainly because of the difficulty associated with breaking the body's immunological tolerance to the antigen. These antigens lack the inherent immunogenicity associated with bacterial antigens and, therefore, methods to enhance immunological recognition and induction of immunity in vivo are under investigation. These include defining the appropriate tumour-associated antigen, successfully synthesizing the antigen to mimic the original molecule, inducing an immune response, and subsequently enhancing the immunological reactivity so that all components can work together. This has been successfully accomplished with several glycolipid and glycoprotein antigens using carriers such as keyhole limpet haemocyanin (KLH) together with a saponin adjuvant, QS-21. Not only can high titre IgM and IgG antibodies be induced, which are specific for the antigen used for immunization, but the antibodies can mediate complement lysis. The approaches for synthesis, conjugation, clinical administration and immunological potential are discussed.

Comparison of antigen constructs and carrier molecules for augmenting the immunogenicity of the monosaccharide epithelial cancer antigen Tn

We have demonstrated previously that the optimal method for inducing an antibody response against defined cancer antigens is covalent conjugation of the antigen to keyhole limpet hemocyanin (KLH) and use of the potent saponin adjuvant QS-21. Single molecules of glycolipids (tetrasaccharides, pentasaccharides, or hexasaccharides) and MUC1 peptides (containing between one and five MUC1 tandem repeats) conjugated to KLH have proven sufficient for antibody recognition and vaccine construction. However, cancer specificity of monoclonal antibodies against the monosaccharide Tn and disaccharide sTn comes largely from recognition of clusters (c) of these molecules on the cell surface. Tn consists of a monosaccharide (GalNAc) O-linked to serine or threonine on epithelial cancer mucins which are uniquely rich in serines and threonines. We test here several Tn constructs: Tn monosaccharide, Tn(c) prepared on a triple threonine backbone, and Tn prepared on a partially or fully glycosylated MUC1 backbone. We determine that Tn(c) is more effective than Tn, and conjugation to KLH is more effective than conjugation to BSA or polystyrene beads for inducing ELISA reactivity against Tn, and FACS reactivity against Tn-positive tumor cells. Surprisingly, MUC1 glycosylated with Tn at three or five sites per 20 amino acid MUC1 tandem repeat and conjugated to KLH, induced the strongest antibody response against Tn and tumor cells expressing Tn, and had the additional advantage of inducing antibodies against MUC1.

Identification of a New, Testis-Specific Sperm Antigen Localized on the Principal Piece of the Spermatozoa Tail in the Fox (Vulpes vulpes)1

Fox (Vulpes vulpes) sperm antigens were identified to assess them as a potential target for a contraceptive vaccine. We report here the cloning and sequencing of fSP13, a fox sperm protein of 97 kDa. The fSP13 protein was both auto- and iso-antigenic in foxes; it was recognized by sera of foxes immunized with fox sperm proteins and vasectomized foxes. The NH2-terminal sequence of fSP13 was determined, and a piece of cDNA was amplified from testicular RNA by reverse transcription polymerase chain reaction. This piece was used to screen a cDNA library from fox testis by Southern blot. A sequence of 1662 base pairs was obtained, including a major open reading frame coding for 498 amino acid. Mass spectrometry analysis confirmed the position of the open reading frame and the presence of posttranscriptional modifications. Analysis of the predicted amino acid sequence revealed no apparent transmembrane regions. Comparison of the protein sequence with the Prosite database demonstrated the presence of four potential N-linked glycosylation sites. The fSP13 bears the closest amino acid similarity to two human sperm proteins: fibrousheathin 2 and testis-specific calcium binding protein 86-VII. The deduced 80 N-terminal amino acid sequence also presents similarity with the RIIalpha domain. By using a serum against fSP13, this antigen was localized on the principal piece of the fox spermatozoa. Northern blot analysis showed that fSP13 is specifically expressed in testis. The fSP13 is one of the first fox sperm antigens to be cloned and sequenced.

Bioengineering of Surface GD3 Ganglioside for Immunotargeting Human Melanoma Cells

N-Propionyl, N-butyryl (N-Bu), and N-benzoyl mannosamine, as precursors of sialic acid biosynthesis, were incubated with human melanoma SK-MEL-28 cells and resulted in the replacement of N-acetyl groups on the cell surface sialic acid residues, including those associated with GD3. Meanwhile, vaccines containing GD3 and modified GD3 tetrasaccharide-keyhole limpet hemocyanin conjugates were synthesized, and BALB/c mice were immunized with them together with monophosphoryl lipid A adjuvant. The GD3Bu-keyhole limpet hemocyanin conjugate raised the highest IgG titers without any cross-reactivity to unmodified GD3. Expression of GD3Bu epitopes on the surface of SK-MEL-28 cells was confirmed in vitro and in vivo by the binding of a polyclonal antiserum and monoclonal antibody (mAb) 2A, both of which specifically recognize GD3Bu, and by mass spectroscopic analysis of glycolipids extracted from cells. Following expression of GD3Bu on the surface of SK-MEL-28 cells, the cells could be lysed by mAb 2A and GD3Bu antiserum in the presence of complement. Although less effective in the control of existing large size tumors ( approximately 10 mm inner diameter) on BALB/c nu/nu mice, mAb 2A in combination with ManNBu effectively protected mice from SK-MEL-28 tumor grafting. This approach may provide a method to augment the immunogenicity of sialylated human antigens and to avoid generating an autoimmune response to them at same time.

Fully synthetic carbohydrate-based vaccines in biochemically relapsed prostate cancer: clinical trial results with alpha-N-acetylgalactosamine-O-serine/threonine conjugate vaccine

PURPOSE

We report the synthesis of a mucin-related O-linked glycopeptide, alpha-N-acetylgalactosamine-O-serine/threonine (Tn), which is highly simplistic in its structure and can induce a relevant humoral response when given in a trimer or clustered (c) formation. We tested for an antitumor effect, in the form of a change in the posttreatment versus pretreatment prostate-specific antigen (PSA) slopes, that might serve as a surrogate for effectiveness of vaccines in delaying the time to radiographic progression.

METHODS

We compared the antibody response to immunization with two conjugates, Tn(c)-keyhole limpet hemocyanin (KLH) and Tn(c)-palmitic acid (PAM) with the saponin immunologic adjuvant QS21, in a phase I clinical trial in patients with biochemically relapsed prostate cancer. Patients received Tn(c)-KLH vaccine containing either 3, 7, or 15 microg of Tn(c) per vaccination. Ten patients received 100 microg of Tn(c)-PAM. QS21 was included in all vaccines. Five vaccinations were administered subcutaneously during 26 weeks with an additional booster vaccine at week 50.

RESULTS

Tn(c), when given with the carrier molecule KLH and QS21, stimulated the production of high-titer immunoglobulin M (IgM) and IgG antibodies. Inferior antibody responses were seen with T(c)-PAM. There was no evidence of enhanced immunogenicity with increasing doses of vaccine. An antitumor effect in the form of a decline in posttreatment versus pretreatment PSA slopes was also observed.

CONCLUSION

A safe synthetic conjugate vaccine in a trimer formation was developed that can break immunologic tolerance by inducing specific humoral responses. It seemed to affect the biochemical progression of the disease as determined by a change in PSA log slope.

Preparation of synthetic glycoconjugates as potential vaccines against Shigella flexneri serotype 2a disease

The synthesis of three neoglycopeptides incorporating carbohydrate haptens, differing in length, covalently linked to a non natural universal T helper peptide is disclosed. They were synthesized according to a blockwise strategy based on the condensation of appropriate di-, tri-, and tetrasaccharide trichloroacetimidate donors onto an azidoethyl 2-acetamido-2-deoxybeta-D-glucopyranoside acceptor. Use of thiol-maleimide coupling chemistry allowed site-selective efficient conjugation.

Construction and characterization of a Pseudomonas aeruginosa mucoid exopolysaccharide-alginate conjugate vaccine

Deterioration of lung function in patients with cystic fibrosis (CF) is closely associated with chronic pulmonary infection with mucoid Pseudomonas aeruginosa. The mucoid exopolysaccharide (MEP) from P. aeruginosa has been shown to induce opsonic antibodies in mice that are protective against this chronic infection. MEP-specific opsonic antibodies are also commonly found in the sera of older CF patients lacking detectable P. aeruginosa infection. When used in a human vaccine trial, however, MEP only minimally induced opsonic antibodies. To evaluate whether conjugation of MEP to a carrier protein could improve its immunogenicity, we bound thiolated MEP to keyhole limpet hemocyanin (KLH) by using succinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate (SMCC) as a linker. In contrast to the native MEP polymer, the MEP-KLH conjugate vaccine induced high titers of MEP-specific immunoglobulin G (IgG) in C3H-HeN mice and in a rabbit. Sera from mice immunized with MEP-KLH conjugate, but not from animals immunized with comparable doses of native MEP, demonstrated opsonic killing activity. Vaccination with MEP-KLH conjugate induced opsonic antibodies broadly cross-reactive to heterologous mucoid strains of P. aeruginosa. Preexisting nonopsonic antibodies to MEP are found in normal human sera, including young CF patients, and their presence impedes the induction of opsonic antibodies. Induction of nonopsonic antibodies by either intraperitoneal injection of MEP or injection or feeding of the cross-reactive antigen, seaweed alginate, reduced the level of overall IgG elicited by follow-up immunization with the MEP-KLH conjugate. However, the opsonic activity was lower only in the sera of MEP-KLH conjugate-immunized mice with preexisting antibodies induced by MEP but not with antibodies induced by seaweed alginate. Immunization with MEP-KLH elicited a significant proportion of antibodies specific to epitopes involving O-acetate residues, and this subpopulation of antibodies mediated opsonic killing of mucoid P. aeruginosa in vitro. These results indicate that conjugation of MEP to KLH significantly enhances its immunogenicity and the elicitation of opsonic antibodies in mice and rabbits, that the conjugate induces opsonic antibodies in the presence of preexisting nonopsonic antibodies, and that opsonic antibodies to MEP are directed at epitopes that include acetate residues on the uronic acid polymer.

Imparting bone affinity to glycoproteins through the conjugation of bisphosphonates

PURPOSE

To develop a novel means of conjugating bisphosphonates onto the carbohydrate moieties of glycoproteins to enhance protein affinity to bone.

METHODS

1-Amino-1,1-diphosphonate methane (aminoBP) was conjugated onto the carbohydrate moietites of oxidized fetuin by using 4-(maleimidomethyl)cyclohexane-1-carboxyl-hydrazide (MMCCH). Bone affinity of the resulting conjugates was compared to proteins obtained from another means of conjugation, whereby aminoBP was conjugated onto fetuin's lysine moieties by using succinimidyl-4-(N-maleimidomethyl)-cyclohexane-1-carboxylate (SMCC).

RESULTS

The use of the MMCCH resulted in the conjugation of up to seven aminoBPs per molecule of fetuin. These conjugates gave a 2.6-, 2.0-, 30.5-. and 1.84-fold increased affinity for untreated, ashed, demineralized bone and hydroxyapatite, respectively, as compared to conjugates from the SMCC reaction. Both conjugates exhibited a pH-independent, equally slow degradation in adult bovine serum-containing media.

CONCLUSION

The use of the MMCCH chemistry to conjugate aminoBP onto fetuin was feasible. Furthermore, the described processes of conjugation resulted in amino-BP-dependent increase in the glycoprotein's affinity to various bone matrices in a manner that exceeds the affinity produced by the previously established method, which used SMCC.

Synthesis of maleimide-activated carbohydrates as chemoselective tags for site-specific glycosylation of peptides and proteins

An array of maleimide-activated mono- and oligosaccharides were synthesized to permit site-specific glycosylation of cysteine-containing peptides and proteins. Maleimide-activated monosaccharides, in which the native alpha- or beta-O-glycosidic linkages found for nonreducing terminal sugars of native glycoproteins are preserved, were prepared using 2'-aminoethyl glycosides as the key intermediates. In addition, a native high-mannose type oligosaccharide, Man(9)GlcNAc(2)Asn, was converted into its maleimide-activated form by taking advantage of the existing amino group in the Asn portion. The application of these maleimide-activated carbohydrates was exemplified by the site-specific glycosylation of a 36-mer HIV-1 gp41 peptide, T20, which is a potent inhibitor against HIV infection. The chemoselective ligation was found to be rapid, highly efficient, and essentially quantitative. Tagging the biologically active peptide with a mannose and/or oligomannose moiety will be useful for targeting the drug to macrophage and dendritic cells, which are primary targets for HIV-1 infection and are expressing mannose- and oligomanose-specific receptors on their surface. In combination with site-specific mutagenesis, the maleimide-activated carbohydrates can serve as generally applicable tags for site-specific glycosylation of proteins via the highly efficient maleimide-thiol ligation reaction.

Constructing an adenocarcinoma vaccine: immunization of mice with synthetic KH-1 nonasaccharide stimulates anti-KH-1 and anti-Le(y) antibodies

There is mounting evidence to suggest that immunization-based strategies can be used to mobilize the human immune system against specific carbohydrate antigens displayed on the surface of cancer cells. Following isolation and identification, such antigens can be administered as conjugate vaccines. The tumor-associated carbohydrate antigen KH-1 is 1 such antigen and may serve as a potential target for immunization against adenocarcinoma. However, a serious impediment to the application of a vaccine-based approach involving this antigen is that its availability from natural sources is severely limited. In order to overcome this limitation, we have developed an efficient total synthesis of this complex glycolipid. We have extended our synthesis to reach a structurally related analog in which the ceramide portion of KH-1 is replaced with an allyl substituent. These synthetic advances have led to the preparation of 2 potential vaccine constructs, each based on the conjugation of the KH-1 nonasaccharide and the carrier protein keyhole limpet hemocyanin (KLH). In 1 construct (KH-1-Et-KLH), the nonasaccharide is conjugated to KLH via a simple ethyl linkage, while in the other (KH-1-MMCCH-KLH), conjugation is mediated by a 4-(4-N-maleimidomethyl)cyclohexane-1-carboxyl hydrazide (MMCCH) cross-linker. We report here the immunological properties of these 2 constructs. Mice were immunized with either of the 2 KH-1-KLH vaccine candidates or the KH-1 ceramide, along with the immunological adjuvant QS-21. Immunization with the ceramide served as a negative control and, as expected, failed to stimulate the production of antibodies against the KH-1 glycolipid. The construct in which the KH-1 nonasaccharide is linked to KLH via a simple alkyl chain stimulated significant quantities of IgM antibodies, whereas the construct linked to KLH by MMCCH induced high titers of both IgM and IgG antibodies. Inhibition data demonstrated that antibodies generated in response to immunization with the KH-1-KLH constructs recognize not only the KH-1 antigen but also the Lewis(y) (Le(y)) antigen, which, from a structural perspective, is similar to the 4 residues located at the non-reducing end of the KH-1 nonasaccharide. Thus, the KH-1-KLH constructs elicit an immune response that successfully targets 2 adenocarcinoma markers. As assessed by FACS analysis, the antibodies raised were strongly reactive with the KH-1/Le(y) positive cell line MCF-7 but not with KH-1 and Le(y) negative melanoma cell lines. Based on the results of our study, a KH-1-KLH plus QS-21 vaccine is being prepared for clinical evaluation.

Novel Tn antigen-containing neoglycopeptides: synthesis and evaluation as anti tumor vaccines

The fully unprotected alpha-C-glycosyl analogue of N-acetylgalactosamine 9 was conjugated by a non-natural oxime bond to the segment peptides (328--340)OVA and (327--339)OVA, affording neoglycopeptides 1--2 and 3, having one or two sugar units, respectively. The three neoglycopeptides were tested in vitro in an antigen presentation assay as antitumor vaccines. Neoglycopeptides 1--3 could be presented to and recognized by the T cell receptor; neoglycopeptide 3, bearing two B-epitopes, was presented to the TCR with higher efficiency, compared to neoglycopeptide 2, having only one B-epitope.

Influence of terminal residue on adjacent disaccharide immunogenicity

Aberrant O-glycosylation of cell surface mucin antigens is characteristic of epithelial cancer cells. For example, Thomsen-Friedenreich disaccharide (TFD) is a chemically well-defined carbohydrate antigen with a documented link to malignancy. There have been many attempts to improve immune response to carbohydrate antigens, for use in immunotherapy. As part of an alternative strategy to improve carbohydrate immunogenicity, we studied the influence of terminal benzyl (Bzl) or p-nitrophenyl (pNP) residue on immunogenicity of adjacent TFD. Mice immunized with keyhole limpets hemocyanin-TFD (KLH-TFD), KLH-TFD(alpha)Bzl, or KLH-TFD(alpha)pNP produced anti-KLH antibodies, which were analyzed by enzyme-linked immunosorbent assay (ELISA). KLH-TFD did not give significant anti-TFD antibody titer, confirming the poor immunogenicity of TFD. Immunization with KLH-TFD(alpha)Bzl and KLH-TFD(alpha)pNP raised antibody titers against TFD(alpha)Bzl and TFD(alpha)pNP, respectively. KLH-TFD(alpha)Bzl also gave higher anti-TFD antibody response, whereas KLH-TFD(alpha)pNP did not, indicating that terminal Bzl residue improves immune response to adjacent carbohydrate. Analysis of anti-TFD(alpha)Bzl or anti-TFD(alpha)pNP IgG antibodies by competitive ELISA, using carbohydrate-related antigens as inhibitors, demonstrated their high specificity to their respective antigens. Anti-TFD(alpha)pNP antibody was not inhibited by TFD, but was significantly inhibited by GalNAc(alpha)pNP. The fact that p-nitrophenol (pNPol) has more competitive ability that GalNAc indicates that terminal polar residue is the main target antigen. In contrast, anti-TFD(alpha)Bzl antibody was inhibited to a similar degree by GalNAc(alpha)Bzl and TFD, confirming the carbohydrate recognition by antibodies yielded by terminal non-polar modification of the immunogen.

Immunization of metastatic breast cancer patients with a fully synthetic globo H conjugate: a phase I trial

The carbohydrate antigen globo H commonly found on breast cancer cells is a potential target for vaccine therapy. The objectives of this trial were to determine the toxicity and immunogenicity of three synthetic globo H-keyhole limpet hemocyanin conjugates plus the immunologic adjuvant QS-21. Twenty-seven metastatic breast cancer patients received five vaccinations each. The vaccine was well tolerated, and no definite differences were observed among the three formulations. Serologic analyses demonstrated the generation of IgM antibody titers in most patients, with minimal IgG antibody stimulation. There was significant binding of IgM antibodies to MCF-7 tumor cells in 16 patients, whereas IgG antibody reactivity was observed in a few patients. There was evidence of complement-dependent cytotoxicity in several patients. Affinity column purification supported the specificity of IgM antibodies for globo H. On the basis of these data, globo H will constitute one component of a polyvalent vaccine for evaluation in high-risk breast cancer patients.

Large-scale production of the carbohydrate portion of the sialyl-Tn epitope, alpha-Neup5Ac-(2-->6)-D-GalpNAc, through bacterial coupling

Alpha-Neup5Ac-(2-->6)-D-GalpNAc, the carbohydrate portion of sialyl-Tn epitope of the tumor-associated carbohydrate antigen, was prepared by a whole-cell reaction through the combination of recombinant Escherichia coli strains and Corynebacterium ammoniagenes. Two recombinant E. coli strains overexpressed the CMP-Neup5Ac biosynthetic genes and the alpha-(2-->6)-sialyltransferase gene of Photobacterium damsela. C. ammoniagenes contributed to the production of UTP from orotic acid. Alpha-Neup5Ac-(2-->6)-D-GalpNAc was accumulated at 87 mM (45 g/L) after a 25-h reaction starting from orotic acid, N-acetylneuraminic acid, and 2-acetamide-2-deoxy-D-galactose.

Tumor-associated carbohydrate antigens defining tumor malignancy: basis for development of anti-cancer vaccines

Tumors expressing a high level of certain types of tumor-associated carbohydrate antigens (TACAs) exhibit greater metastasis and progression than those expressing low level of TACAs, as reflected in decreased patient survival rate. Well-documented examples of such TACAs are: (i) H/Le(y)/Le(a) in primary non-small cell lung carcinoma; (ii) sialyl-Le(x) (SLe(x)) and sialyl-Le(a) (SLe(a)) in various types of cancer; (iii) Tn and sialyl-Tn in colorectal, lung, breast, and many other cancers; (iv) GM2, GD2, and GD3 gangliosides in neuroectodermal tumors (melanoma and neuroblastoma); (v) globo-H in breast, ovarian, and prostate cancer; (vi) disialylgalactosylgloboside in renal cell carcinoma. Some glycosylations and TACAs suppress invasiveness and metastatic potential. Well-documented examples are: (i) blood group A antigen in primary lung carcinoma; (ii) bisecting beta1 --> 4GlcNAc of N-linked structure in melanoma and other cancers; (iii) galactosylgloboside (GalGb4) in seminoma. The biochemical mechanisms by which the above glycosylation changes promote or suppress tumor metastasis and invasion are mostly unknown. A few exceptional cases in which we have some knowledge are: (i) SLe(x) and SLe(a) function as E-selectin epitopes promoting tumor cell interaction with endothelial cells; (ii) some tumor cells interact through binding of TACA to specific proteins other than selectin, or to specific carbohydrate expressed on endothelial cells or other target cells (carbohydrate-carbohydrate interaction); (iii) functional modification of adhesive receptor (integrin, cadherin, CD44) by glycosylation. So far, a few successful cases of anti-cancer vaccine in clinical trials have been reported, employing TACAs whose expression enhances malignancy. Examples are STn for suppression of breast cancer, GM2 and GD3 for melanoma, and globo-H for prostate cancer. Vaccine development canbe extended using other TACAs, with the following criteria for success: (i) the antigen is expressed highly on tumor cells; (ii) high antibody production depending on two factors: (a) clustering of antigen used in vaccine; (b) choice of appropriate carrier protein or lipid; (iii) high T cell response depending on choice of appropriate carrier protein or lipid; (iv) expression of the same antigen in normal epithelial tissues (e.g., renal, intestinal, colorectal) may not pose a major obstacle, i.e., these tissues are not damaged during immune response. Idiotypic anti-carbohydrate antibodies that mimic the surface profile of carbohydrate antigens, when administered to patients, elicit anti-carbohydrate antibody response, thus providing an effect similar to that of TACAs for suppression of tumor progression. An extension of this idea is the use of peptide mimetics of TACAs, based on phage display random peptide library. Although examples are so far highly limited, use of such "mimotopes" as immunogens may overcome the weak immunogenicity of TACAs in general.

Induction of antibodies against GD3 ganglioside in melanoma patients by vaccination with GD3-lactone-KLH conjugate plus immunological adjuvant QS-21

The gangliosides GD3, GD2 and GM2 are expressed on the cell surface of malignant melanomas, GD3 being the most abundant. We have shown that immunization of melanoma patients with GM2 adherent to Bacillus Calmette-Guerin (GM2/BCG) induced an IgM antibody response. Vaccines containing GM2-keyhole limpet hemocyanin (KLH) conjugate and the immunological adjuvant QS-21 induced a higher titer IgM response and consistent IgG antibodies. Patients with antibodies against GM2 survived longer than patients without antibody. On the other hand, our previous trials with GD3/BCG, GD3 derivatives including GD3-lactone (GD3-L)/BCG failed to induce antibodies against GD3. In our continuing efforts to induce antibody against GD3, we have immunized groups of 6 melanoma patients with GD3-KLH or GD3-L-KLH conjugates containing 30 microg of ganglioside plus 100 microg of QS-21 at 0, 1, 2, 3, 7 and 19 weeks. Prior to vaccination, no serological reactivity against GD3 or GD3-L was detected. After immunization, IgM and IgG antibodies were detected against both GD3 and GD3-L in the GD3-L group exclusively. The GD3-L-KLH vaccine induced IgM titers against GD3-L of 1:40-1/1,280 in all patients and IgG titers of 1/160-1/1,280 in 4 patients. These antibodies also strongly cross-reacted with GD3. ELISA reactivity was confirmed by immune thin-layer chromatography on GD3 and melanoma extracts. Sera obtained from 4 of these 6 patients showed cell surface reactivity by FACS and from 2 showed strong cell surface reactivity by immune adherence (IA) assay and complement lysis against the GD3 positive cell line SK-Mel-28.

Keyhole limpet hemocyanin (KLH): a biomedical review

In this review we present a broad survey of fundamental scientific and medically applied studies on keyhole limpet hemocyanin (KLH). Commencing with the biochemistry of KLH, information on the biosynthesis and biological role of this copper-containing respiratory protein in the marine gastropod Megathura crenulata is provided. The established methods for the purification of the two isoforms of KLH (KLH1 and KLH2) are then covered, followed by detailed accounts of the molecular mass determination, functional unit (FU) structure, carbohydrate content, immunological analysis and recent aspects of the molecular genetics of KLH. The transmission electron microscope (TEM) has contributed significantly to the understanding of KLH structure, primarily from negatively stained images. We give a brief account of TEM studies on the native KLH oligomers, the experimental manipulation of the oligomeric states, together with immunolabelling data and studies on subunit reassociation. The field of cellular immunology has provided much relevant biomedical information on KLH and has led to the expansion of use of KLH in experimental immunology and clinically as an immunotherapeutic agent; this area is presented in some detail. The major clinical use of KLH is specifically for the treatment of bladder carcinoma, with efficacy probably due to a cross-reacting carbohydrate epitope. KLH also has considerable possibilities for the treatment of other carcinomas, in particular the epithelially derived adenocarciomas, when used as a carrier for carcinoma ganglioside and mucin-like epitopes. The widespread use of KLH as a hapten carrier and generalised vaccine component represent other major on-going aspects of KLH research, together with its use for the diagnosis of Schistosomiasis, drug assay and the treatment of drug addiction. Immune competence testing, assessment of stress and the understanding of inflammatory conditions are other areas where KLH is also making a useful contribution to medical research.

The preparation of neoglycoconjugates containing inter-saccharide phosphodiester linkages as potential anti-Leishmania vaccines

The Leishmania express complex glycoconjugates containing phosphosaccharide repeat units at all stages of their life-cycle. One of these molecules, lipophosphoglycan (LPG) has been suggested to be a vaccine candidate. To assess the immunological properties of Leishmania phosphosaccharides, we have prepared neoglycoproteins and neoglycolipids containing synthetic Leishmania phosphosaccharide repeats. The coupling procedure uses the dec-9-enyl spacer of previously synthesised phosphosaccharides for linkage to protein and phospholipid. This alkene moiety is converted by ozonolysis to an aldehyde which is then attached to protein and phospholipid amino groups by reductive amination. The procedure produces neoglycoconjugates in good yield and without compromising the labile phosphodiester linkages within the phosphosaccharide chains.

Carbohydrate vaccines in cancer: immunogenicity of a fully synthetic globo H hexasaccharide conjugate in man

The complex carbohydrate molecule globo H hexasaccharide has been synthesized, conjugated to keyhole limpet hemocyanin, and administered with the immunologic adjuvant QS-21 as a vaccine for patients with prostate cancer who have relapsed after primary therapies such as radiation or surgery. Globo H is one of several candidate antigens present on prostate cancer cells that can serve as targets for immune recognition and treatment strategies. The vaccine, given as five subcutaneous vaccinations over 26 weeks, has been shown to be safe and capable of inducing specific high-titer IgM antibodies against globo H. Its immunogenicity was confirmed in prostate cancer patients with a broad range of stages and tumor burdens. Observations of several patients who had evidence of disease relapse restricted to a rising biochemical marker, prostate-specific antigen (PSA), indicated that a treatment effect could occur within 3 months after completion of the vaccine therapy. This effect was manifested as a decline of the slope of the log of PSA concentration vs. time plot after treatment compared with values before treatment. Five patients continue to have stable PSA slope profiles in the absence of any radiographic evidence of disease for more than 2 years. The concept of using PSA slope profiles in assessing early treatment effects in biological therapies such as vaccines awaits further validation in phase II and III trials. The use of a variety of lesser known candidate glycoprotein and carbohydrate antigens in prostate cancer serves as a focus for the development of a multivalent vaccine of the treatment of relapsed prostate cancer in patients with minimal tumor burden.